1. Field of the Invention
The present invention is related to balancing/unbalancing structures, or “baluns,” for use in gigahertz wireless applications.
2. Related Art
A balun (short for BALanced to Unbalanced) is a transformer connected between a balanced source or load (signal line) and an unbalanced source or load (signal line). A balanced line has two signal line conductors, with equal currents in opposite directions. The unbalanced signal line has just one conductor; the current in it returns via a common ground or earth path. Typically, an RF balun function is implemented as an off-chip transformer or as a quarter wave hybrid (lumped or microstrip) integrated into an RF circuit board.
RF wireless circuits utilize balanced outputs of signals to minimize the effect of ground inductance and to improve common mode rejection. Circuits that benefit from balanced operation include mixers, modulators, IF strips, differential amplifiers and voltage controlled oscillators. These balanced outputs, moreover, consist of differential signals which must be combined to provide a single ended output signal. Thus, a balun is a RF balancing network or electric circuit for coupling an unbalanced line or device and a balanced line or device for the purpose of transforming from balanced to unbalanced or from unbalanced to balanced operation, with minimum transmission losses. A balun can be used with an unbalanced input and a pair of balanced outputs or, in the reverse situation, a pair of balanced sources and an unbalanced load. Baluns can be used to interface an unbalanced input with a balanced circuit by dividing the signal received at its unbalanced terminal equally to two balanced terminals, and by providing the signal at one balanced terminal with a reference phase and the signal at the other balanced terminal with a phase that is 180° out-of-phase relative to the reference phase. Plus or minus 180° baluns can be used to interface a balanced or differential input from a balanced port of a balanced circuit providing output signals which are equal in magnitude but 180° out-of-phase and an unbalanced load driven by a single-ended input signal. The balun combines the signals of the balanced input and provides the combined signal at an another port.
The balanced structure can improve performance in devices such as mixers, modulators, attenuators, switches and differential amplifiers, since balanced circuits can provide better circuit-to-circuit isolation, dynamic range, and noise and spurious signal cancellation. A balanced load is defined as a circuit whose behavior is unaffected by reversing the polarity of the power delivered thereto. A balanced load presents the same impedance with respect to ground, at both ends or terminals. A balanced load is required at the end of a balanced structure to ensure that the signals at the balanced port will be equal and opposite in phase.
Depending on the implementation, baluns can be divided into two groups: active and passive. Active baluns are constructed by using several transistors (so-called active devices). Although active baluns are very small, they are not generally preferred for the following reasons. First, due to the employment of active devices, noise will be introduced into the system. Also, active devices tend inherently to waste power. Additionally, the low-cost fabrication of active baluns is limited to semiconductor manufacture. Conversely, passive baluns are quite popular. Passive baluns include lumped-type baluns and distributed-type baluns.
Lumped-element-type baluns employ discrete components that are electrically connected, such as lumped element capacitors and lumped element inductors. Advantages of lumped-element-type baluns include small size and suitability for low frequency range usage. On the other hand, the performance of lumped-element-type baluns is not good in high frequency ranges (several GHz), because the lumped elements are very lossy and difficult to control. Also, the operational bandwidth of lumped-element-type baluns is small (<10%, typically).
A 180° hybrid device is constructed from several sections of quarter-wavelength transmission lines and a section of half-wavelength transmission line. The drawbacks of the 180° hybrid device are larger size, difficulty in achieving a high impedance transformation ratio, and limitation to a balanced pair of unbalanced outputs.
In general, low return loss, low insertion loss, and good balanced characteristics are required for balun applications. In addition, bandwidth is another figure of merit.
An example of a conventional 180° hybrid is shown in FIG. 1, which shows four hybrids, all of which have a rat-race arrangement. The hybrids are suitable for 5.3 GHz operation, and a single hybrid is shown in FIG. 2, along with representative dimensions. As may be seen from FIG. 2, the footprint of each hybrid is approximately 473 mm2 (18.2×26 mm), including the feeding arms, and the overall size of the board in FIG. 1, which includes the four hybrids, is about 2916 mm2 (54×54 mm). As shown in FIG. 3, the hybrid of FIGS. 1 and 2 may be thought of as a 3-port microwave device, with an input port, and two output ports, one of which outputs the signal with a phase of 0° at −3 dB, and the other one outputs the signal at 180°, at −3 dB. It will be appreciated that for a passive device such as illustrated in FIGS. 1 and 2, the designation of “input” or “output” is purely arbitrary. In practical applications, the single-ended input (or output) may, for example, be connected to an antenna, while the differential output (or input) may be connected to a differential amplifier, or differential driver.
However, many of the known passive balun structures are relatively large, which is often unacceptable in modem wireless applications.